Over the past several years, HRL Laboratories of Malibu, Calif. has developed the concept of the tunable impedance surface, which can be used for electronically steerable antennas. A new application has for this technology emerged, in which very lightweight antennas are needed, for which a tunable impedance surface is well qualified. However, this particular application requires independent two-frequency operation, and the tunable impedance antennas proposed to date do not provide for independent multiple frequency operation. In this disclosure, we describe how two-frequency operation (and, more generally, multiple frequency operation) can be obtained with a tunable impedance surface. This invention provides simultaneous electronic steering in both (or all) bands. It is an improvement of the prior art tunable impedance surface concepts, it is thin and lightweight, and ideally suited to the application for which it was designed, to be described below. The technology described herein in terms of two frequency operation can be expanded to allow multiple band operation with independent beam steering in each band, so long as the bands are sufficiently separated from one another (they need be spaced at least an octave apart).
This invention represents an improvement over prior art tunable impedance surfaces, because it is capable of providing electronic beam steering in two (or more) frequency bands independently and simultaneously. In the past, dual band high-impedance surfaces have been studied, but these were not tunable. Using these previous designs, it would not be possible to tune both bands independently. This invention provides independent tuning in both bands, as long as the two bands are separated by at least one octave in frequency.
This antenna could be used as part of a large stratospheric airship for remote sensing. Because the antenna is based on the tunable impedance surface concept, it is thin compared to the wavelength of interest. If made of lightweight materials, as described below, it can be light enough that even large area antennas (tens or hundreds of square meters) can be carried on a lighter-than-air craft that can be operated in the stratosphere.
The closest prior art is that of tunable impedance surfaces, and dual band high impedance surfaces. The prior art includes the patents listed below:
R. Diaz, W. McKinzie, “Multi-Resonant High Impedance Electromagnetic Surfaces”, U.S. Pat. No. 6,774,867.
W. McKinzie, S. Rogers, “Multiband Artificial Magnetic Conductor”, U.S. Pat. No. 6,774,866.
W. McKinzie, V. Sanchez, “Mechanically Reconfigurable Artificial Magnetic Conductor”, U.S. Pat. No. 6,690,327.
R. Diaz, W. McKinzie, “Multi-Resonant High-Impedance Surfaces Containing Loaded Loop Frequency Selective Surfaces”, U.S. Pat. No. 6,670,932.
J. Hacker, M. Kim, J. Higgins, “High-Impedance Structures for Multifrequency Antennas and Waveguides”, U.S. Pat. No. 6,628,242.
D. Sievenpiper, T-Y Hsu, S-T Wu, D. Pepper, “Electronically Tunable Reflector”, U.S. Pat. No. 6,552,696.
D. Sievenpiper, R. Harvey, G. Tangonan, R. Loo, J. Schaffner, “Tunable Impedance Surface”, U.S. Pat. No. 6,538,621.
W. McKinzie, “Reconfigurable Artificial Magnetic Conductor Using Voltage Controlled Capactors with Coplanar Resistive Biasing Network”, U.S. Pat. No. 6,525,695.
R. Diaz, W. McKinzie, “Multi-Resonant High-Impedance Electromagnetic Surfaces”, U.S. Pat. No. 6,512,494.
D. Sievenpiper, J. Schaffner, “Textured Surface Having High Electromagnetic Impedance in Multiple Frequency Bands”, U.S. Pat. No. 6,483,481.
D. Sievenpiper, G. Tangonan, R. Loo, J. Schaffner, “Tunable Impedance Surface”, U.S. Pat. No. 6,483,480.
FIG. 1(a) depicts a prior art single-band tunable impedance surface 5, both in a plan view and in a side section view, which consists of an array of metal patches 10 that are connected by tunable capacitors, such as varactor diodes 15, arranged above a conductive ground plane 12. The metal patches 10 are connected alternately to the ground plane 12 or to a set of control lines 17 through a sheet of dielectric material 19 disposed between the metal plates 10 and the ground plane 12. When a voltage is applied to the control lines 17, the resonance frequency of the surface is tuned, and this effect can be used to steer a reflected radio frequency (RF) beam.
The FIG. 1(b) is a graph of exemplary curves showing the reflection phase as a function of frequency for different control voltages for the tunable surface of FIG. 1(a). For a frequency within the tuning range of the surface, nearly any desired phase can be produced by applying the correct control voltages to the control lines 17.
When a pattern of voltages is applied to the control wires, the tunable capacitors are tuned to a pattern of capacitance values. The reflection phase of the surface depends on the value of the capacitors, and is also a function of frequency. The pattern of capacitances results in a pattern of reflection phases. By tuning the surface to create a phase gradient, a reflected wave is steered to an angle that depends on the phase gradient.
Therefore, the tunable impedance surface of FIG. 1(a) may be used as a beam steering reflector as shown in FIG. 2(a) where an incoming RF beam is reflected at a desired angle as a reflected RF beam. A phase gradient is created using the tuning method described above. A wave reflected by the surface is steered to an angle that depends on the phase gradient. FIG. 2(b) depicts the measured beam steering results of the single band surface shown in the previous figures. The different radiation patterns correspond to different sets of control voltage applied to the control lines. Using this reflective beam steering method, the tunable surface is typically fed using a free-space feed method, such as a horn antenna that is set apart from the surface.
FIG. 3(a) shows an alternative method of feeding the tunable surface, with a conformal feed. This technique is used when the entire antenna must occupy a short height, and a space feed either cannot be used or is not desired. Beam steering is more difficult with this feed technique, but it eliminates the need for a space feed. In this case, the feed is a small antenna 7 such as a dipole, located near the surface. The feed excites surface waves in the surface. The surface waves propagate across the surface, and radiate to form a beam in a direction that depends on the pattern of control voltages applied to the tunable capacitors. FIG. 3(b) depicts an example of a measured radiation pattern using the direct feed method shown in FIG. 3(a). The beam is broad because the surface is small. Many such tiles can be combined to make a narrow, steerable beam without the need for a space feed.
The present invention is described in the context a dual-band tunable impedance surface in which both bands are independently tunable. It is based on, and an improvement of, the prior art tunable impedance surface designs, which are described in the patent documents identified above. It is capable of dual band operation through the use of a different principle than the prior art multi-band surfaces. The design can be extended to so that more than two bands can be independently tunable.
This present invention is useful for applications where antennas that are capable of independent beam steering in two different frequency bands are required. It is particularly useful for air or space based structures, where lightweight structures are important. In particular, such an antenna could be used in stratospheric airships, which must be lightweight.